In battery-powered systems, the ability to accurately estimate the charge remaining in the battery is highly desirable, and in many cases essential. For example, in portable electronic devices such as cameras, cell phones, portable gaming systems and computers, knowing such information allows the end user to gauge how much longer they can use the device before recharging becomes necessary. In some cases, this information can prevent the end user from inadvertently losing data, a common occurrence when a camera or a computer suddenly stops functioning due to the battery becoming fully discharged. In other applications, such as electric vehicles, knowing the remaining battery capacity may make the difference between a successful trip and an unsuccessful trip, i.e., one in which the vehicle and its driver become stranded when, without providing sufficient warning, the battery becomes fully discharged. Additionally, since a battery's voltage drops as the state of charge of the battery is reduced, knowing the state of charge allows an accurate estimate to be made of the power available to the battery-operated device, e.g., an electric vehicle.
In order to accurately estimate the remaining capacity of a battery, it is critical that the full capacity of the battery be accurately known. Unfortunately, under normal use conditions such as those encountered in an electric vehicle or other battery-power device, it is difficult to accurately ascertain battery capacity. For example, in one method of determining battery capacity, the initial capacity of the battery is gradually decreased based on a variety of factors such as battery age, the number of charge/discharge cycles to date, and temperature. Unfortunately this technique does not provide a very accurate assessment of battery capacity, both because some factors are not properly taken into account (e.g., historical temperature profiles, load conditions, depth of discharge prior to each charging, charge/discharge rates, etc.) and because the effects of the errors accumulate as the battery ages. Another method of determining battery capacity is to allow the battery to become fully discharged, and then determine the capacity of the battery during charging. Although this technique can be used occasionally, using it on a routine basis can have serious repercussions since deep discharging a battery, and in particular fully discharging a battery, can dramatically shorten its lifetime. Additionally, for most battery-powered devices, especially electric vehicles, it would be extremely inconvenient to require that the user allow the battery to become fully discharged prior to charging. This would be similar to requiring that a conventional car be driven until the gas tank was dry before refilling, simply in order to determine the gas tank's capacity.
Accordingly, what is needed is a method of accurately estimating battery capacity that is less susceptible to, or more accurately takes into account, the many factors that can influence a battery's capacity. The present invention provides such an estimation method and apparatus for implementing the same.